This invention relates to the testing, calibrating and proving of flowmeters and more particularly relates to improvements in small volume "ballistic type" meter provers for the testing of flowmeters.
Conventional flowmetering devices of the positive displacement or turbine types are used in the installation and operation of fluid transmission systems for the handling of various liquids. Because flowmeters are subject to serious inaccuracies which may be cumulative, it is common to use meter provers to make accurate periodic checks of the flow for calibration of the flowmeter. To maintain accurate readings from a flowmeter, it therefore must be calibrated from time to time to determine its K-factor, i.e., the constant of proportionality between the flow rate of the fluid flowing through the flow meter and the response given by the flowmeter.
In the case of a turbine type flowmeter, electrical pulses are developed by the flowmeter which are proportional in number to the volume of fluid flow through the flowmeter. The K-factor is expressed in terms of the number of pulses generated by the flowmeter per unit volume of fluid passing through the flowmeter, and is a function of the type of fluid, as well as the fluid temperature, pressure, and flow rate, and varies as the parts of the flowmeter wear in the course of use.
API Standard 2531 establishes the tests for certification of a flowmeter. Such certification is required when custody transfers of fluids are made. The API code requires a discrimination of one part in ten thousand, i.e., there is an uncertainty of plus or minus one pulse in a total of 10,000 meter pulses generated by the flowmeter per volume of liquid displaced. This achieves an accuracy of one part in 10,000. Further, five meter prover runs are normally required with a repeatability of 0.02% between the runs.
One type of conventional meter prover involves propelling a solid body, such as a spherical resilient plug, through a given section of a conduit arranged in series with the flowmeter to be proved. The conduit is provided with a detection means for indicating when the plug moves past an initial position and a final position. The volume of the conduit and therefore the fluid displacement between these positions is carefully calibrated. By recording the volume of fluid flowing through the flowmeter during the time it takes the plug to flow the distance between the two positions, the flowmeter reading may be checked against the known conduit volume. Such meter provers are disclosed in U.S. Pat. Nos. 2,772,561; 3,021,703; 3,397,570; and 3,738,153.
The initial concept of apparatus for calibrating flowmeters as disclosed in U.S. Pat. Nos. 2,772,561; 3,021,703; and 3,397,570, for example, depends entirely on pulses from the flowmeters for discrimination in measuring discrete units of volume. To obtain the meter calibration accuracy possible with the use of such equipment, the accumulation of 10,000 or more pulses from the meter during a single proving run became standard, and was adopted by the American Petroleum Institute as one of the criteria for meter prover design in the API 2531 "USA Standard for Mechanical Displacement Meter Provers." A volume of 0.5% of the maximum hourly flow rate of the meter being proved was also adopted as a suggested volume between detectors for the design of the meter prover, and consideration was given to the resolution of the detectors in the prover design. A system repeatability of 0.02%, the discrimination of the electronic counter which counted the pulses generated by the meter, and a maximum velocity of the prover displacer of 10 feet per second were all suggested targets for the design of meter proving systems in the API 2531 code.
Plug-type meter provers have received widespread use because the meter prover can be made as long as necessary to permit a sufficiently large displacement volume for the flow meter to emit 10,000 meter pulses as required by the API code. However, requiring the meter prover to be large enough to handle such a displacement volume dictates that the meter prover be very bulky and stationary. Emphasis by the trade is now being placed on a smaller and more portable meter prover.
The new API "Manual of Petroleum Measurement Standards, Chapter 4 ," which supersedes the API 2531, no longer places a restriction on minimum volume between detectors or displacer velocity, which has assisted in the acceptance of small, compact, low volume displacement, high speed meter prover designs. These provers have become known by the generic name of "ballistic" provers. This name originated with the ballistic calibrator patented by E. E. Francisco, Jr., U.S. Pat. No. 3,403,544 and marketed by Flow Technology, Inc. The term "ballistic" prover is now commonly used to describe small, low volume displacement meter provers. With the removal of volume and displacer velocity restrictions in the current code, the basic targets for design of meter proving equipment remaining are a 0.02% system repeatability, 10,000 or more pulses per proving run, and detector resolution. Since the volume defined by the meter prover detector (or pair of detectors in conventional meter proving equipment) can be calibrated to a volume repeatably within 0.02%, and the meter delivers 10,000 pulses (representing 10,000 discrete units of volume) during a proving run, the performance of the meter is measured to two parts in 10,000.
The ballistic meter prover employs a piston that travels in a cylinder in synchrony with the fluid traveling through the flowmeter. By measuring the time interval required for the piston to travel a given distance through the cylinder, an average flow reading can be calculated and compared to the pulses of the flowmeter. This comparison is then used to determine the K-factor.
A typical ballistic meter prover is disclosed in U.S. Pat. No. 4,152,922. The prover includes a measuring cylinder having an inlet and an outlet at its ends connected in a fluid system in series with a flowmeter. A piston, having a passage therethrough, is adapted to travel through the cylinder as a fluid barrier when a poppet valve on the inlet side of the piston closes the passage through the piston. A hydraulic actuator rod is connected to the poppet valve and piston for actuating the poppet valve and retracting the piston within the measuring cylinder. Detectors are mounted on the actuator cylinder of the actuator rod to define the volume displacement of the piston. During a prover run, control circuitry actuates the actuator rod to close the poppet valve in the piston. Fluid pressure at the inlet of the cylinder causes the piston to travel through the measuring cylinder, and the actuator rod, affixed to the piston, actuates the detectors as the rod passes through the actuator cylinder.
Various features of ballistic provers are shown in U.S. Pat. Nos. 3,273,375; 3,403,544; 3,492,856; and 4,152,922, in Waugh Controls Corporation Bulletin PB700.1 entitled "Model 700 MicroProver," and in an article entitled "Inline Liquid Flow Prover" by Richard E. Zimmermann of Flow Technology, Inc. The '375, '544, '856, and '922 patents and the Waugh publication disclose a meter prover having a main cylinder with an inlet and outlet connected in series with a flowmeter in a fluid line. The piston is shown sealing with the main cylinder as it passes through the cylinder during the proving run in the '544, '856, and '922 patents and Waugh publication. The '922 patent discloses a lip seal disposed around the periphery of the piston upstream of the 0 ring seal. The '856 and '922 patents disclose a valve within the piston, and the '922 patent shows the use of O rings between the poppet valve and piston for sealing. A hydraulic piston return is shown in the '922 patent and Waugh publication, and as discussed previously, the '922 patent shows a hydraulically actuated poppet valve. The '922 and '375 patents show piston spacers or stops at each end of the cylinder. Other spacers are shown in the '544 and '856 patents and the Waugh publication.
The Waugh publication discloses a leak detector for the piston seals. Two seals are disposed around the piston, separated by a small circumferential chamber. There is a passageway through the piston to a flexible tube which extends to the downstream end of the cylinder where the tube is connected to a solenoid valve and a pressure switch which is designed to fire on rising pressure. The solenoid valve dumps the pressure between the piston seals to atmospheric (or to a pressure lower than that in the operating prover) and the valve closes. Any seal leakage will increase the pressure in the conduit containing the pressure switch and blocked by the closed solenoid valve, causing an increase in pressure and a corresponding leak indication by the pressure switch. The Waugh publication also discloses a double block and bleed ball-type bypass valve having both upstream and downstream seals with a bleed port leading to the space between the seals. The valve is connected to a solenoid valve and pressure switch as described for the piston seals to warn of any seal leakage past the valve.
The prior art shows the use of multiple detectors. The '375, '544, and '856 patents and the Waugh publication show multiple detectors mounted on the main cylinder. The detector on the Waugh publication is magnetic, and the detector in the '375 patent is pressure actuated.
Other patents show the detectors associated with the hydraulic actuator system for the piston. The '922 patent discloses multiple detectors mounted on the actuator cylinder. The '856 patent discloses the piston rod extending outside the cylinder and attached to a detection system.
U.S. Pat. No. 3,830,092 discloses a leak detection system for a sphere-type meter prover. Two oppositely faced cup-shaped sealing members are secured back to back for sealing engagement with an inner cylindrical surface of a sleeve. The rims of the cup-shaped sealing members contact the cylindrical surface to form an annulus. The flexing of the rims of the sealing members causes the pressure inside the annulus between the sealing members to be substantially less than the adjacent fluid pressure. A pressure monitoring system is connected to the annulus to register this difference in pressures. So long as a pressure differential is indicated, there is no leakage through the sleeve.
Ballistic meter provers are, in general, smaller and more portable than conventional sphere-type meter provers. Most ballistic meter provers are too small to have a sufficient fluid capacity for a volume displacement between detectors which will permit 10,000 pulses from the flowmeter during the prover run. Efforts are being made to adopt a chronometry system where an oscillator is connected to the flowmeter to generate hundreds of oscillations per flowmeter pulse. Although the API code does not sanction the chronometry system, many customers, who do not require certification, are utilizing such a system.
A double chronometry system is disclosed in the Zimmermann article and in the '544 patent. As discussed in the Zimmermann article, for example, as the piston passes through the first and second detector, pulses from a 100 kilohertz oscillator are counted to determine the time of travel of the piston over a given displacement volume. This establishes the rate of flow. As the piston passes the first detector, a second counter also begins counting the same oscillator pulses. This counter does not stop when the second detector is passed, but keeps counting until the next flowmeter pulse has been completed. Since the oscillator frequency is many times higher than the flowmeter pulse frequency, the time interval between the whole pulses of the flowmeter precisely determines the flowmeter frequency. The K-factor is then determined by multiplying the ratio of the flowmeter pulse count over the displacement volume times the ratio of the first oscillator pulse count over the second oscillator pulse count times 60. The ratio of the first oscillator to second oscillator compensates for the fraction of a meter pulse which would introduce an error in a conventional prover.
A bell prover is another type of prover used for gas meters. Such a prover is discussed in an article published in the August 1970 issue of Pipeline and Gas Journal entitled "Proving Large Capacity Meters to Meet High Accuracy Demands" by David F. Kee. A bell prover includes an outer tank and an inner cylindrical dry well forming an annulus therebetween. The annulus is filled with a fluid medium such as oil. The bell is suspended so as to be telescopingly received by the annulus between the tank and dry well. Gas is introduced into the dry well causing the bell to move upwardly. The increased gas volume is then measured by a scale against the upward rise of the bell. This provides a convenient and accurate linear displacement analog of the newly delivered volumetric quantity of gas.
The prior art uses multiple detectors in their meter provers. However, because one detector may drift so that it will not trip at exactly the same point through repeated meter prover runs, a true calibrated volume may not be maintained between the detectors, thereby introducing error into the calibration run. Many meter provers have doubled up on the number of detectors and use four detectors. This is redundant and increases the complexities of the system.
In the use of a plurality of detectors, if one detector has to be removed, repaired, or changed, the displacement volume has to be recalibrated. Such a recalibration requires that the prover be drained of fluid, cleaned, and then recalibrated using certified containers for water drawing. This may be very tedious task because it frequently has to be done in the field, such as at offshore rigs, or for large stationary provers at pipeline stations or refineries.
The ballistic provers of the prior art are unidirectional provers. The piston moves in only one direction for a prover run and then must be returned for another run, i.e., the provers do not displace in both directions as bidirectional sphere provers do.
For large turbine meters, and meters of low resolution (pulse output), a considerable amount of volume is displaced by the meter during the generation of 10,000 pulses. For example, a typical 12-inch turbine meter generates 250 pulses per barrel throughput; therefore, in order to generate 10,000 pulses during a proving run, the prover detector or detectors must define a minimum volume of 40 barrels.
Most turbine meters in use today use a variable reluctancetype pickup coil mounted on the meter to detect rotational movement (angular velocity) of the bladed rotor. The coil does not intrude into the flowing stream, and a voltage is generated by the coil assembly as each blade of the turbine passes through the coil's magnetic field. Some meters use a shroud around the turbine blades which is perforated, or in which magnetic "buttons" are installed so as to increase the number of voltage pulses generated per rotation of the turbine blade. There is a practical limit to the spacing of the reluctance slots or magnetic buttons to allow the focusing of magnetic flux and the generation of sufficient voltage to be sensed by the meter's associated electronic counter. These systems are very efficient, as the turbine assembly is designed to "float" during normal operation, and the slight magnetic drag of the reluctance pickup is minimal and consistent. These meters are capable of a linearity of .+-.0.15% and a repeatability of .+-.0.02% over extended periods of time.
Efforts to increase the resolution of these meters have traditionally sacrificed accuracy. Turbine meters of many varieties are now manufactured where the turbine blade drives a pulsing means through a direct gear connection. This gearing must be inserted into the flowing stream, and the output shaft to the external pulsing means must be sealed. This usually results in meter performance of .+-.1.5% linearity and a repeatability of .+-.0.2%, a reduction of meter performance by a factor of ten.
The purpose of this invention is to take advantage of the accuracy and repeatability of low resolution meters and yet prove these meters (establish an accurate K-factor) with a small volume displacement prover by delivering a very large number of pulses (representing very small discrete units of volume) generated by the prover which can be related to the pulse output of the meter, thus reversing the current concept of volumetric proving.
In this invention, only a single detector is used to define the calibrated volume, and since any variation in the switching action at the beginning of the proving run can be expected to repeat at the end of the proving run, the effects of detector resolution no longer exist. Therefore, the criterion remaining is to repeatably define the volume displaced in a meter prover when the meter under test has delivered a minimum of 10,000 pulses.
The present invention provides an accurate, simple and quick way to determine the K-factor of a flowmeter. The prover may be of any size so as to permit the required 10,000 pulses of the API code, or can be adapted to use a chronometry system. The prover may be compact and portable. Further, it is much less expensive.
Other objects and advantages of the invention will appear from the following discription.